The invention relates generally to polymer composites. More specifically, to polymer composites capable of absorbing electromagnetic interference signals. The invention also relates to articles comprising polymer composites capable of absorbing electromagnetic interference signals.
Electrical and electronic devices are often sensitive to electromagnetic interference (EMI) that is present in the environment due to many sources. EMI sources may be self-induced due to noisy components (e.g. transistors), or externally induced by electronic equipment (e.g., fluorescent lighting, motors, radar transmitters). Electromagnetic energy may propagate by conduction through wires or conductive surfaces or through reactive coupling (inductive or capacitive-crosstalk between wires), or radiation. EMI often disrupts the device performance and may lead to total failure.
To reduce these problems, it is a common practice to shield the devices from external EMI. Conventionally, metal sheets such as Faraday cages are disposed around electrical systems to reflect the electromagnetic waves and to prevent the system from unwanted signals. However, they are bulky, not very effective at higher frequencies, and are difficult to form into highly convoluted shapes. Alternatively, electromagnetic absorbing layers are used, which could be used as a coating inside a Faraday box to dissipate any energy that enters rather than to allow it to build up inside of the resonant structure. Commonly used absorbing materials are based on either magnetic or conducting particles disposed within a polymer matrix. Dielectric or magnetic materials have intrinsic impedance different from that of free space (377 Ohms), which causes an electrical mismatch at the air/shield interface. The electrical mismatch may mitigate efficient EMI absorption. Conducting particles, on the other hand, contribute to the conductivity of the absorbing layer and may lead to electrical shortage or shock. Alternatively, certain absorber materials are being developed based on low dielectric constant (i.e., permittivity) materials. These materials are only effective in attenuating EMI at very large thicknesses. In spite of considerable effort, there is still a need for electromagnetic interference shielding that effectively operates at higher frequencies, is compact, thin, lightweight, and is suitable for wide frequency bands. Further, there is a need for simpler and versatile methods to prepare these materials for use in electromagnetic interference shielding.